Polished or plated metal has been used for many years to provide durable and decorative surfaces in a number of applications. For example, chromium metal has been used to provide such a bright decorative surface to designated parts such as car bumpers and trim. Chromium is, however, being designed out of current applications because of its weight, uncertain availability and expense.
In a number of applications, other bright polished metals such as aluminum can provide a similar appearance to chromium while avoiding the weight, expense and uncertain availability associated with chromium. Aluminum is abundantly available, relatively light in weight, durable, and forms a protective coating of aluminum oxide about 50 angstroms thick which makes it highly resistant to ordinary corrosion.
Aluminum is, however, susceptible to attack by acids and bases, and polished aluminum surfaces do not resist weathering, but instead develop a milky appearance that generally results from the aluminum oxide coating. Because of aluminum's advantages in weight and durability, however, a number of techniques have been developed to attempt to protect aluminum, and these techniques have met with varying degrees of success.
One technique is chemical anodization which provides improved protection and the option of color tinting. Anodization, however, has not been demonstrated to result in long-term protection on exposed parts under many environmental conditions, and the use of the process itself is under increasing environmental pressure because of the problems associated with waste disposal requirements for the spent chemicals used in the surface anodization process. Anodization also tends to embrittle the aluminum, limiting the extent to which post-anodization forming techniques can be successfully used.
Another technique for protecting aluminum is the direct coating of the aluminum with a polymer. This generally provides some advantages over anodization in terms of protection. Typical polymer coating processes, however, require extensive solvent handling, followed by baking or curing of the polymer after its application. Furthermore, polymer coatings often either are or become brittle, may delaminate, and may show "orange peel," "cracking", "crazing", or "blushing" effects after exterior exposure. Coatings such as the clear vinyl coatings which are sometimes used to face a bright metal can also mold, mildew or stain.
Yet another technique is the lamination of a polymeric film to an aluminum surface using an adhesive. Such laminates can demonstrate some of the same theoretical advantages as polymer coatings. Typically, however, such laminates lack clarity, do not exhibit long-term bond durability, and exhibit poor formability. As will be understood to those familiar with the working of decorative metals, formability means that the finished metal may be worked, whether by stamping, pulling, or bending, without affecting the decorative surface. As is known to chemists, metallurgists, and engineers, one of the advantages of metals is their malleability, ductility, and flexibility. Accordingly, coatings or treatments of metals which will not withstand such metal-working techniques are inappropriate, disadvantageous, or even useless, depending upon the needed application. Other problems which such a surface treatment should address include adhesion, and resistance to heat, water, solvent, mechanical scrubbing and biological attack.